Catalytic depolymerization of Kraft lignin to high yield alkylated-phenols over CoMo/SBA-15 catalyst in supercritical ethanol

Lignin is generally considered to be a renewable and sustainable resource of aromatic chemicals. However, the depolymerization of Kraft lignin (KL) for the production of selective phenolic monomers presents a significant challenge due to its highly recalcitrant nature. Therefore, in this work, we investigated the effect of metal sites and acid active sites on Mo/SBA-15, Co/SBA-15 and CoMo/SBA-15 catalysts in supercritical ethanol for the depolymerization of KL to produce phenolic monomers. Ethanol was used as a hydrogen donor solvent instead of using external hydrogen. Results showed that the bimetallic CoMo/SBA-15 catalyst exhibited significantly higher catalytic activity compared to the monometallic, Co/SBA-15, Mo/SBA-15 or bare SBA-15. The highest phenolic monomers yield of 27.04 wt% was achieved at 290 °C for 4 h over CoMo/SBA-15 catalyst. The inter-unit linkages such as β-O-4′, β–β and α-O-4′ in lignin were considerably cleaved during the catalytic depolymerization in supercritical ethanol. Meanwhile, higher functionality of carbonyl compounds was present in the non-catalytic bio-oil, while more alkylated phenols were produced over CoMo/SBA-15 catalyst. The major phenolic monomers identified in the catalytic bio-oil were 4-ethylguaiacol (9.15 wt%), 4-methylguaiacol (6.80 wt%), and 4-propylguaiacol (2.85 wt%). These findings suggest that the metal sites and abundant acid active sites of CoMo/SBA-15 had a synergistic effect toward the degradation of different linkages of lignin and production of selective phenolic monomers in bio-oils.


Introduction
Lignin is a major constituent of lignocellulosic biomass that accounts for 10-30% of its weight and contributes to approximately 40% of its energy content. 1,2 Among different lignins, Kra lignin (KL) is generated as an industrial byproduct in pulp mills, with an estimated annual production exceeding 40-50 million tons worldwide. 3,4 Despite the abundant availability of KL compared to fossil fuels, a signicant portion of it remains underutilized, oen being used as low-grade fuel in recovery boilers, leading to severe environmental concerns. Notably, KL holds promise as a potential source of high-grade chemicals if it can be converted into smaller units. However, the main challenge associated with the valorization of KL is its highly variable structure. The chemical compositions of different available KLs depend on their botanical origin, extraction techniques and delignication methods. Therefore, to address the signicant heterogeneity of KL and enhance the economic viability of Kra mills, signicant efforts have been made to depolymerize KL into various value-added chemicals.
To date, several approaches have been developed for the depolymerization of lignin including fast pyrolysis, 5 oxidation, 6-8 solvothermal, 9,10 and hydrothermal liquefaction. 11,12 Of these approaches, solvothermal has received signicant attention as a technique for lignin degradation, which involves the use of external hydrogen or a hydrogen donor solvent with or without the presence of a metal catalyst. Supercritical alcohols such as methanol, 13,14 ethanol 15,16 or isopropanol 17,18 are being widely used for lignin depolymerization due to their favorable hydrogen donor behavior, excellent solubilization capabilities and efficient heat transfer. Taking into account the positive role of supercritical alcohols, Warner et al. 14 studied the depolymerization of organosolv lignin (source: candlenut nutshells) in supercritical methanol using Cu-and La-doped hydrotalcite-like catalysts and achieved a complete lignin conversion into bio-oil at 310°C for 6 h. Compared to methanol or isopropanol; ethanol is proven to be an effective solvent for the production of high yield of phenolic monomers and the suppression of repolymerization during the lignin depolymerization reaction. For example, Feng et al. 19 studied KL (source: extracted from the wheat straw biomass) using NiAl/MCM-41 catalyst at different temperatures (250-310°C ), times (30-90 min), and solvents (water, ethanol and methanol). They achieved a bio-oil yield of 56.2 wt% and reported that ethanol is more effective in preventing repolymerization than other solvents. In our previous work, 1 we investigated concentrated acid hydrolysis lignin (source: black liquor) liquefaction in water and organic solvents (such as methanol, ethanol, and isopropanol), and found that ethanol gave less char formation and high monomers yield as compared to those obtained with other solvents. Based on the ndings presented above, ethanol can be considered as one of the best performing solvent for lignin conversion to phenolic monomers under supercritical conditions. In recent years, there has been increasing interest in Co-and Mo-based catalysts supported on various carriers for the hydrogenolysis of lignin and the enhancement of bio-oil quality. 9,[20][21][22] These catalysts have attracted attention due to their crystalline structures and ability to exist in multiple valence states. For example, Wang et al. 22 evaluated the effect of Mo/sepiolite catalyst on the depolymerization of KL (Indulin AT) in supercritical ethanol and it was concluded that the presence of Lewis active sites and Mo species (Mo 6+ and Mo 5+ ) facilitated the deoxygenation and alkylation reactions during lignin depolymerization, resulting in the production of phenolic monomers. Zhang et al. 23 compared the role of Co/ MCM-41, Co/SBA-15 and Co/FDU-1 catalyst on the hydrodeoxygenation of vanillin (a lignin-derived model compound) and found that Co/SBA-15 had the best catalytic activity. Yang et al. 24 reported that Co/SBA-15 catalyst could selectively hydrogenolysis the C-O bond in the lignin and can acts as a hydrodeoxygenation catalyst of lignin-derived bio-oils, due to its high efficiency in the removal of oxygen by a direct hydrogenolysis route. Besides excellent catalytic activities of monometallic catalyst, bimetallic catalysts offer the possibility to accelerate the lignin hydrogenolysis process due to their combined effect. Considering the benecial effect of bimetallic catalyst, Pu et al. 25 used CoMoS/Al 2 O 3 catalyst for the depolymerization of protobind 1000 lignin (source: wheat straw soda lignin) in a semi-continuous reactor at 350°C for 13 h and they obtained a 62.5 wt% yield of organic products. The same group 26 also developed a kinetic model for the hydroconversion of protobind 1000 lignin over CoMoS/Al 2 O 3 catalyst in a semibatch reactor. Hao et al. 27 studied the catalytic performance of the Co-Mo-S/AC, Co-Mo-S/g-Al 2 O 3 , and Co-Mo-S/ZrO 2 catalysts on the depolymerization of dealkaline lignin (source: needle-leaved trees and broad-leaved trees) and obtained 95.76 wt% liquid products at 340°C for 2.5 h over Co-Mo-S/ ZrO 2 catalyst. Jongerius et al. 28 developed a two-steps process for the depolymerization of various lignins, including KL (source: Indulin AT) to produce monomeric compounds over Pt/g-Al 2 O 3 catalyst in water/ethanol system and the achieved bio-oil was further deoxygenated with CoMo/Al 2 O 3 catalyst. However, a common challenge faced by researchers in the eld of bio-reneries is the low surface area and potential catalyst deactivation due to the clogging of support material. Therefore, the choice of a suitable metal carrier or support is also crucial in determining the catalytic activity of catalysts.
Currently, mesoporous silicate materials such as MCM-41, Al/MCM-41 and SBA-15 have gained attention as supports/ carriers due to their high surface area, excellent thermal stability and adjustable pore sizes. For example, Kim et al. 29 evaluated the effect of supercritical ethanol over Ru-Ni/SBA-15 catalyst for the depolymerization of soda lignin (source: wheat straw and sarkanda grass) into phenol-rich bio-oil. They obtained 77.5 wt% yield of bio-oil at 350°C for 40 min. Lu et al. 30 investigated lignin (source: Chinese r sawdust) depolymerization in isopropanol/formic acid solvent mixture over NiMo/ Al-MCM-41 catalyst, and attained a bio-oil yield of 61.6 wt%. In another study, Klamrassamee et al. 31 33 prepared a Co/CaO catalyst for the depolymerization of KL (Sigma-Aldrich, product no. 471003) in water, ethanol and methanol solvents. They reported a maximum biooil yield of 60.2 wt% in methanol using 10 wt% Co/CaO, highlighting the excellent catalytic capability of Co for the hydrodeoxygenation of depolymerized products of lignin. Chen et al. 34 examined KL (Sigma-Aldrich, product no. 471003) depolymerization into small-molecular liquid products in supercritical ethanol over Mo 2 N/Al 2 O 3 catalyst. Dou et al. 35 prepared a ZnCoOx catalyst for the KL (Sigma-Aldrich, product no. 471003) depolymerization in a mixed solvent of methanol and 1,4-dioxane. They achieved a 72% yield of liquid products at 300°C for 6 h and reported the essential role of Co to the cleavage of ether linkages in lignin via hydrogenolysis. Although, KL-471003 has been widely utilized for the aromatic chemicals, so far, research has not been reported on the depolymerization of KL (product no. 370959) into aromatic chemicals. In a recent study, Ghoreishi et al. 36 investigated depolymerization of lignin extracted from various sources such as Norway Spruce-BL, Eucalyptus and Alkali lignin, in waterethanol-formic acid and showed that the yield and composition of liquid products obtained aer each lignin depolymerization are quite different. Therefore, extensive research is required for each lignin species, as lignin structure and its depolymerization products greatly depend on the plant types and the lignin isolation methods.
Inspired by the intriguing catalytic properties of Co-and Mobased catalysts and the SBA-15 support, this study focused the effects of mono and bifunctional Mo/SBA-15, Co/SBA-15 and CoMo/SBA-15 catalysts on the depolymerization of commercially available KL (product no. 370959) to produce phenolic monomers in supercritical ethanol. Different reaction parameters such as temperature and time were investigated to determine the optimal conditions. The prepared catalysts were characterized via BET, XPS, XRD and HR-TEM analyses, while the produced bio-oils were analyzed by GC-MS, FT-IR, 13 C NMR, 2D HQSC NMR, and elemental analyses.

Preparation of catalyst
The catalyst support porous SBA-15 material was synthesized according to our previous work. 37 Herein, 10 g of P123 was added to 120 mL of 2 M aqueous HCl solution and 30 mL of deionized (DI) water and stirred at room temperature for 15 min. Subsequently, 20 g of TEOS was added to the solution and the mixture was heated at 40°C with a moderate stirring speed of 300 rpm for 2 days. The resulting solution was then transferred to a Teon-lined autoclave and heated at 100°C for 2 days. Finally, the solid product was ltered, washed multiple times with DI water, dried in air at 105°C overnight and calcined in a muffle furnace at 550°C for 4 h.
For the preparation of CoMo/SBA-15 catalyst, the porous SBA-15 was impregnated with aqueous solutions of (NH 4 ) 6 -Mo 7 O 24 $4H 2 O (0.39 mmol) and Co(NO 3 ) 2 $6H 2 O (1.68 mmol). Cobalt and molybdenum loadings in the SBA-15 were xed at 5 wt%, respectively, (total metal loading = 10 wt% and Co : Mo ratio = 1 : 1). The impregnated precursors were then stirred at 60°C for 2 h followed by prolonged drying at 100°C for 12 h. Finally, the resulting solid product was calcined at 550°C (10°C min −1 ) for 4 h under static air conditions. For comparison, monometallic Co and Mo catalysts supported on SBA-15 (denoted as Co/SBA-15 and Mo/SBA-15, respectively) were also prepared by an incipient wetness impregnation method according to the similar method described above.

Lignin depolymerization
All experiments were performed in a 150 mL batch reactor (Hastelloy-C-276 HR-8300) equipped with a reactor controller (Parr 4848). Initially, 3.0 g of KL and 90 mL of ethanol with or without SBA-15 supported metal catalysts were loaded into a stainless-steel reactor. The reactor was then sealed and purged with N 2 to expel the air inside. The reactor was heated to the intended reaction temperature at a rate of 10°C min −1 by an electric furnace, and the reactor was maintained at this temperature for the specied reaction time. When the preset reaction conditions were completed, the reactor was cooled to room temperature in an ice-water bath. Finally, the reactor was vented and opened to collect the desired reaction products. Fig. 1 shows the detailed product separation techniques. The reaction mixture was rst ltered to separate the liquid fraction from the solid product. The solid product was washed with ethanol and then dried in an oven at 110°C and weighed. The solid residue/product was composed of unreacted lignin, char and catalyst. Meanwhile, the liquid fraction was evaporated in a rotary evaporator at a reduced pressure and 60°C to remove the ethanol solvent and to obtain bio-oil, a brown liquid. The bio-oil thus obtained was weighed and collected for further analysis. Lignin-derived bio-oil typically consists of a volatile fraction (monomers) and a non-volatile fraction (oligomers). To extract the volatile fraction, 20 mL of ethyl acetate solution was added to the bio-oil, followed by centrifugation and ltration. The resulting ethyl acetate soluble fraction was subjected to rotary evaporation to obtain the volatile fraction, referred to as light-oil. While the ethyl acetate insoluble products was dened as heavy-oil. The yields of various products were calculated using the following eqn (1-5): Gas (wt%) = 100 − (bio-oil + solid product) where, W BO = weight of the bio-oil, W L = weight of the feed lignin, W SP = weight of the solid product, W Cat = weight of the feed catalyst, W LO = weight of the light-oil and W HO = weight of the heavy-oil. Each experiment was repeated three times and the average value is presented in the result.

Characterization of catalyst
The textural properties of the synthesized catalysts, such as surface area and pore size distribution were determined by conducting N 2 adsorption-desorption isotherms measurements at 77.35 K using a BELCAT-A analyzer. The surface area was determined using the Brunauer-Emmett-Teller (BET) method, while the pore size distribution was calculated using the Barrett-Joyner-Halenda (BJH) method, based on the adsorption branches of the nitrogen adsorption-desorption isotherm. Prior to testing, all samples were degassed in a vacuum (10 −6 Torr) at 200°C for 12 h. The total pore volume was measured at a relative pressure (P/P o ) of 0.990.
The powder X-ray diffraction (XRD) patterns of the synthesized catalysts were obtained using a 3D high resolution X-ray diffractometer (model: EMPYREAN) with a PIXcel 3D , PREFIX INTERFACE Xenon proportional detector and Cu Ka radiation (l = 1.5406 Å) in the 2q ranges of 0.5-5°(for low angles) and 5-90°(for wide angles). The XRD data were qualitative analyzed using High Score Plus 4.1 soware (analyzed with the latest ICDD Card Database). Transmission electron microscopy (TEM) micrographs were captured with a JEOL JEM-2100F electron microscope operated at an acceleration voltage of 200 kV. The elemental compositions of the synthesized catalysts were measured with an EDS detector (ATW2, 133 eV, 10 mm 2 ). An Xray photoelectron spectroscopy (XPS) analysis of the assynthesized catalysts was performed using a PerkinElmer PHI-1600 spectrometer with monochromatic Mg Ka radiation.
Temperature-programmed desorption (TPD) experiments were conducted using NH 3 -TPD to determine the acidity of the samples. The catalysts were initially heated to 500°C and then cooled down to 100°C under a He atmosphere. Subsequently, 5% NH 3 /He (20 mL min −1 ) was adsorbed at 100°C for 30 min, followed by He purging at the same temperature for 1 h. The desorption of ammonia was monitored in the temperature range of 100-900°C at a heating rate of 10°C min −1 . Additionally, ammonia desorption in the samples was analyzed using a thermal conductivity detector (TCD). Inductively coupled plasma-optical emission spectrometry (ICP-OES) was carried out to determine the metals content in the prepared catalyst.

Product analyses
The qualitative and quantitative analyses of bio-oils were conducted using an Agilent 6890 gas chromatograph-mass spectrometer (GC-MS) equipped with an HP-5MS capillary column (30 m × 0.25 mm × 0.25 mm). High purity helium (He) was used as the carrier gas (at a ow rate of 1 mL min −1 ). The column temperature was initially set to 40°C for 2 min and then increased to 170°C at a heating rate of 10°C min −1 , where it was held for 5 min. Finally, the temperature was further increased to 300°C at a heating rate of 10°C min −1 and maintained for an additional 2 min. The individual components in the bio-oils were identied by comparing their GC-MS spectra and retention times with those of authentic compounds available in the National Institute of Standards and Technology (NIST) mass spectral library. For quantication of the yield of selected phenolic compounds (phenol, guaiacol, 4-methylguaiacol, 4-ethylguaiacol, 4-propylguaiacol, eugenol, vanillin, acetovanillone, isoeugenol, and homovanillic acid), an external calibration method was used. The yield was calculated as follows: Yield of monomer (wt%) = the weight of monomer/the weight of lignin × 100 (6) The NMR analyses ( 1 H and 13 C) of raw KL and lignin oils were carried out at 300.9 K using the AVANCE III HD (400.0 MHz) spectrometer manufactured by Bruker (Germany). DMSOd 6 was used as the solvent. The NMR tubes had a diameter of 5 mm, and the pulse delay was set to 1 s with a spectral width of 24 038.5 Hz. HSQC NMR analyses of KL and the ESP fractions were performed using the same NMR analyzer mentioned above. Fourier-transform infrared spectroscopy (FT-IR) analyses were carried out using a 400 FT-IR spectrometer (PerkinElmer, USA). The spectra were logged at a resolution of 1 cm −1 within the wavelength range of 500-4000 cm −1.
The elemental analyses of the feed KL, heavy-oil and light-oil fractions of bio-oils were carried out a Vario MACRO cube analyzer (EL III, Germany). The elemental components, such as C, H, and N of lignin and lignin-derived bio-oils, were analyzed by the elemental analyzer, while the oxygen (O) content was calculated based on the difference. The high heating values (HHV) or caloric values of KL and lignin oils were calculated according to DIN 51900 standard (eqn (7)): (7) 3. Results and discussion    9 prepared Co-Mo-S/AC and Co-Mo-S/ZrO 2 catalysts for the depolymerization of dealkaline lignin. In the XRD pattern of CoMo-S/AC catalyst, they also did not found the peak of Co-Mo-S and reported that the loaded metals were distributed evenly and well over AC support. In the present study, the XPS analysis was further carried out to conrm the deposition of Mo and Co, and the successful synthesis of the SBA-15 material.  Table 1. The unsupported SBA-15 material has a specic surface area of 658.51 m 2 g −1 , a total pore volume of 0.90 cm 3 g −1 , and an average pore diameter of 5.50 nm. When Mo or Co was   Table 1. The parent SBA-15 support showed two broad desorption peaks in the temperature range of 160-300°C (indicating weak acid sites) and 650-830°C (indicating strong acid sites) with values of 0.05 and 0.64 mmol g −1 , respectively. Upon the incorporation of Mo and Co into the SBA-15 material, a decrease in the peak intensity of the strong acid sites and a signicant increase in the peak intensity of weak acid sites were observed. Previous research has reported that metal ions loaded onto an acidic support can act as Lewis acid sites, 41 thus, the peak observed at 160-300°C can be attributed to the presence of Lewis acid sites. Among the prepared catalysts, CoMo/SBA-15 catalyst exhibited the highest total acidity (T = 1.14 mmol g; sum of 1.10 mmol g −1 weak acid and 0.33 mmol g −1 strong acid), which was followed by the Mo/ SBA-15 (T = 1.11 mmol g; sum of 0.48 mmol g −1 weak acid and 0.63 mmol g −1 strong acid) catalyst (Table 1).

Characterization of prepared catalysts
TEM and EDS analyses were carried out to investigate the surface modications that occur before and aer the incorporation of metals into the SBA-15 material. Fig. 4 displays the HR-TEM images and EDS element mapping of the SBA-15 and CoMo/SBA-15 materials. The well-ordered pore channels are clearly discernible in the HR-TEM image of the synthesized SBA-15 material, which is in accordance with the nitrogen adsorption-desorption isotherms and low angle XRD patterns. In the case of the CoMo/SBA-15 material, the dark zones over and outside of the mesoporous structure indicating the deposition of metal particles on the surface of SBA-15. Moreover, the TEM image show that the average particle sizes of the loaded metals on the bulk SBA-15 support was 6.2 nm. While the EDS element mapping results conrmed the presence of Mo and Co particles in the CoMo/SBA-15 material. Furthermore, the EDS analysis of CoMo/SBA-15 (Fig. 4e) indicates that the Co and Mo particles are well dispersed on the SBA-15 material.

Effect of reaction temperature and time
The effect of reaction time on the yield of depolymerized lignin products was performed in ethanol by varying the time from 0.5 h to 6 h at 290°C without catalyst, and the results are illustrated in (Fig. 5a and Table S2 †). As observed, at the initial reaction time of 0.5 h, the yield of bio-oil was 30.08 wt% which included 10.26 wt% light-oil, indicating that lignin was partially depolymerized at shorter reaction time. With increasing the time from 0.5 h to 4 h, the yield of total bio-oil and light-oil improved to 47.05 and 29.17 wt%, respectively, while the solid residue yield decreased from 63.12 to 41.50 wt%. As the reaction time increased further from 4 h to 6 h, the yield of solid residue was found to be increased to 45.06 wt% while the light-oil decreased to 25.39 wt%. These results indicate that longer reaction time (more than 4 h) could favor the repolymerization  of reactive monomers, resulting in high yield of char formation. The FT-IR results of the solid residues obtained at different reaction times (Fig. S4a of the ESI †) also support well the above statement. As depicted in Fig. S4a, † the FT-IR spectrum of the solid residue obtained at 0.5 h showed slight differences with the KL, indicating that the KL depolymerization was insignicant at shorter reaction time. As time grew up, the strength of hydroxyl group (3658-3028 cm −1 ), aromatic rings (1606 and 1128 cm −1 ) and C-O group (1037 cm −1 ) began to weaken obviously. The peaks of the aforementioned groups were less intense in the solid residue obtained at 4 h reaction time. However, as the time further increased to 6 h, the peak intensities of hydroxyl group and aromatic rings were found to be increased again, which gave a good indication for the occurrence of repolymerization at a longer reaction time (>4 h). Huang et al. 42  The effect of reaction temperature on the yield of depolymerized lignin products was investigated in the temperature range of 240-320°C at 4 h reaction time without catalyst. We note that when reaction temperature was varied from (240 to 320)°C, the pressure value inside the reactor ranged from 64 to 130 bar, which was under the pressure limit (up to 490 bar) of this hydrothermal reactor. In Fig. 5b and Table S2, † it can be clearly seen that at low temperature of 240 and 260°C, the yields of solid residue were 61.02 wt% and 56.50 wt%, respectively. These results demonstrate that repolymerization reaction could dominate over the depolymerization reaction under low temperature of 260°C. This observation is also consistent with a previous report, 42 where alkali lignin was depolymerized in ethanol using CuMgAlOx catalyst at the temperature range of 200-350°C and it was reported that recondensation reactions dominate at relatively low temperature (<250°C), while depolymerization reactions become more signicant at higher temperature (>250°C). In the present work, with increasing temperature from 260 to 290°C, the yields of total bio-oil and light-oil fraction increased to 47.05 and 29.17 wt%, respectively, while the solid residue yield decreased to 41.50 wt%. However, with a further increase in temperature from 290 to 320°C, the yields of total bio-oil and light-oil were found to decrease while the solid residue yield increased. At higher temperature (>290°C ), the increase in solid residue yield could be attributed to the condensation or repolymerization of reactive species. This phenomenon was also observed in a previous work studied by Feng et al.,19 who investigated the depolymerization of lignin in ethanol at the temperature range of 250-310°C and concluded that temperature above 300°C accelerated the formation of char. In the present study, the FT-IR spectra of solid residues obtained at different temperatures (Fig. S4b †) also gave further evidence that high temperature (>290°C) favored the condensation or repolymerization of reactive monomers during the KL depolymerization process. From Fig. S4b, † it can be seen that when temperature was increased from 240°C to 290°C, the peak intensities of most of the functional groups were found to be decreased substantially. However, at 320°C, the signal of aromatic ring and hydroxyl groups begin to strong, which could be due to the condensation or repolymerization of reactive monomers. Nakamura et al. 46 investigated the condensation of guaiacol, methylguaiacol and coniferyl alcohols at 250°C and reported that phenolic aromatic rings and unsaturated sidechain carbons (-CH]CH-) are the most reactive species towards repolymerization. Therefore, considering the maximum yields of bio-oil and light-oil and less solid residue, the temperature 290°C was chosen as the optimal reaction temperature for the depolymerization of KL in ethanol.  Table S2 † show the yields of depolymerization products obtained from KL with or without catalyst in ethanol at 290°C and 4 h. The results indicate that all catalysts exhibited better performance in terms of total bio-oil and lightoil yields compared to the non-catalytic reaction. Without catalyst, the yields of solid residue and light-oil were 41.50 and 29.17 wt%, respectively. When SBA-15 was used as the catalyst, the light-oil yield increased to 32.60 wt%, and the solid residue yield decreased to 38.50 wt%, suggesting that SBA-15 played a role in preventing the repolymerization of reactive intermediates. The better catalytic performance of SBA-15 during lignin depolymerization reaction was also observed in Chen et al. 45 work, where it was reported that SBA-15 effectively suppressed repolymerization reaction due to its large pore size and wellordered pore structure. When Co/SBA-15 and Mo/SBA-15 are inserted, the product yields are found to be greatly varied. This can be attributed to the deposition of Mo or Co, which provides metal active sites and increases the acidity of SBA-15 (Table 1). However, Co/SBA-15 exhibits a less signicant effect in terms of bio-oil yield and reduced solid residue production compared to Mo/SBA-15 catalyst. This is because the acidity of Co/SBA-15 is relatively lower than those of the Mo/SBA-15 catalyst. The presence of metal active sites and the moderate increase in SBA-15 acidity collectively accelerate the cleavage of main linkages (b-O-4 ′ ) in lignin, resulting in the production of monomeric products. Interestingly, when Mo/SBA-15 or Co/SBA-15 was replaced by CoMo/SBA-15, the bio-oil and light-oil yields further increased to 56.62 and 48.20 wt%, respectively, while the solid residue yield decreased to 28.50 wt%. This result indicates that the presence of Co and Mo on the SBA-15 framework had a synergistic effect on the hydrogenolysis, hydrogenation and deoxygenation of large lignin fragments, leading to improve bio-oil yield and reduced char formation. Compared to the Mo/ SBA-15 and Co/SBA-15 catalyst; CoMo/SBA-15 showed superior performance in terms of reducing solid residue yield. This can be attributed to the larger pore size and higher acid strength in the CoMo/SBA-15 catalyst (Table 1), enabling the adsorption and further degradation of larger lignin fragments into smaller units. This phenomenon is supported by Kim et al. 29 work, where it was reported that the large pore size of SBA-15 has positive effect on the suppression of char. Therefore, CoMo/ SBA-15 catalyst was found as optimal catalyst which shows the highest yield of light-oil and lowest yield of solid residue due to presence of metal sites and acid active sites in CoMo/SBA-15 catalyst. Table 2 shows the yields of phenolic monomers and oligomers present in the light-oil and heavy-oil fractions of bio-oils obtained from KL depolymerization in ethanol at 290°C for 4 h over different catalysts. It was found that the light-oil fraction of bio-oil contained a large number of monomers (11.56-27.04 wt%), while no signicant yields of monomers were identied in the heavy-oil fraction (0.98-1.77 wt%). In the lightoil fraction of non-catalytic bio-oil (at 290°C and 4 h), the yield of monomers was 11.56 wt%. Aer the treatment with SBA-15, the yield of monomers increased to 13.67 wt%. The increase of monomers yield with SBA-15 could be related to its wellordered porous structure which helps to further 3.5 Bio-oil characterization 3.5.1 GC-MS analysis. GC-MS analysis was performed to determine the compositions of monomeric products presented in the light-oil and heavy-oil fractions of bio-oil derived from the non-catalytic and catalytic depolymerization of KL in ethanol at 290°C for 4 h. Fig. S5-S7 † show the mass spectra of some compounds with their standard ones. The GC detected 24 kinds of phenolic monomers. The yields of major monomers and their distribution is shown in Fig. 6a and b. Notably, bio-oils derived from lignin mainly comprised guaiacyl (G)-type phenolics, suggesting that the utilized KL consisted primarily of G-type units. In the light-oil fraction of non-catalytic bio-oil (Fig. 6a), a range of phenolic derivatives, such as guaiacol (2.85 wt%), 4-methylguaiacol (0.83 wt%), 4-ethylguaiacol (1.33 wt%), eugenol (1.04 wt%), isoeugenol (1.25 wt%), homovanillyl alcohol (0.12 wt%) and homovanillic acid (1.16 wt%) were observed. Such compounds are formed from the cleavage of b-O-4 linkages within the guaiacyl units in lignin. Apart from these phenolics, substantial amounts of vanillin and ethyl vanillate were also detected. In a previous study, Kim et al. 47 investigated the effect of sub-and supercritical ethanol on the depolymerization of organosolv lignin. They reported that under supercritical conditions, ethanol acts as a hydrogen donor solvent, facilitating the hydrodeoxygenation and subsequent hydrogenation of oxygenated phenolic monomers. In the present study, when Mo/SBA-15 was used as a catalyst, the yields of eugenol, isoeugenol and homovanillic acid in the light-oil fraction of bio-oil were found to have decreased, while the yields of 4-propylguaiacol, homovanillyl alcohol and 4-ethylguaiacol were increased simultaneously. These ndings indicate that 4-propylguaiacol may be the hydrogenated product of eugenol and isoeugenol, whereas homovanillyl alcohol and 4ethylguaiacol may be the reductive and decarboxylated products of homovanillic acid (HVA). It is expected that the Mo metal sites and acid sites within SBA-15 facilitate the hydrogenation of unsaturated monomers (e.g. eugenol and isoeugenol) as well as the reduction or decarboxylation of oxygenated monomers (such as homovanillic acid). In a related study, Luo et al. 48 investigated the inuence of Ru metal sites and acid active sites of Al 2 O 3 , TiO 2 and ZrO 2 in supercritical ethanol and observed a synergistic effect on the production of phenolic monomers. Chen et al. 45 also reported the positive impact of metal-doped SBA-15 on the production of alkylated phenolic monomers during the treatment of hydrolyzed lignin in ethanol using a Ni/ Al-SBA-15 catalyst. It is worth noted that the yield of mono phenol showed an increasing trend under all catalytic conditions. Interestingly, when the bimetallic CoMo/SBA-15 was applied, the yields of 4-ethylguaiacol, 4-methylguaiacol, homovanillyl alcohol and 4-propyl guaiacol were found to increase signicantly in the light-oil fraction of bio-oil, which indicates that the CoMo/SBA-15 catalyst enhances the alkylation, hydrogenation and decarboxylation reactions of the unsaturated and oxygenated monomers in the bio-oil, leading to a high yield of selectively alkylated phenolic monomers. This is consistent with the moderate acidity of the CoMo/SBA-15 catalyst, which promotes the production of alkyl phenolic monomers over SBA-15 or Mo/SBA-15 catalysts. The resulting alkyl-guaiacols have potential interest as a green solvent in bioreneries or in higher value-added applications. The phenolic monomers identied in the light-oil fraction of bio-oil were signicantly different from the heavy-oil fraction of bio-oils, as shown in Fig. 6b. Moreover, it can be seen that the yield of monomers was very low in the heavy-oil fraction of bio-oil. This nding indicates that heavy-oil fraction of bio-oil was composed of different types of long-chain oligomeric compounds. Among the monomers, gigantol, (S)-(+)-5-sec-butyl-2-pyrimidinol and 1,1 ′ -2,2 ′ -bis[2,3dimethylbenzoquinonyl were the primary compounds found in the heavy-oil fraction of bio-oil over CoMo/SBA-15 catalyst (Fig. 6b). The overall GC-MS results of this work indicate that the bimetallic CoMo/SBA-15 catalyst is effective for the production of high yield of alkylated phenolics in ethanol under supercritical conditions (290°C and 4 h).

Yield of phenolic monomers in bio-oil
3.5.2 FT-IR analysis of Kra lignin and light-oil. The FT-IR analysis was conducted to validate the structural changes occurring before and aer the depolymerization of KL. Fig. 7 shows the FT-IR spectra of raw KL and the resulting bio-oils (light-oil fraction) obtained with or without catalysts and the characteristic peaks are summarized in Table S3. † The vibrational band appeared at 3619-3093 cm −1 can be attributed to the stretching vibration of the -OH groups in phenolic compounds. The bands at 2928 and 2831 cm −1 correspond to the symmetric and asymmetric vibrations of -CH groups attached to aliphatic methyl (-CH 3 ), methylene (-CH 2 -) and methoxy groups (-OCH 3 ). Clearly, the intensities of these bands were found to be increased in both non-catalytic and catalytic bio-oils compared to raw KL. These results indicate that KL was effectively depolymerized into different types of alkyl substituted phenolic compounds through non-catalytic and catalytic reactions. The peak observed at 1733-1712 cm −1 is associated with the carbonyl group (C]O). This peak was also found to have increased in the non-catalytic biooil indicating that under supercritical conditions (290°C and 4 h) without catalyst, ethanol could break the C-O-C and C-C linkages in lignin, resulting in the formation of different phenolic and carbonyl compounds, such as aldehydes, ketones and carboxylic acids. However, with CoMo/SBA-15 catalyst, the carbonyl (C]O) peak intensity was found to be small compared to that of the bio-oil obtained without a catalyst or with Mo/SBA-15. These results are in good agreement with the 13 C NMR spectra of KL and bio-oils as shown in Fig. 8a, which showed a signicant increase in the peak  intensities in aliphatic regions (d C = 0-100 ppm) of methyl, methylene and methane groups in the catalytic bio-oil compared to non-catalytic bio-oil or raw KL. Additionally, the non-catalytic bio-oil showed high-intensity peaks of ether or ester groups, which were reduced in the catalytic bio-oils. The presence of metal sites and abundant acid active sites in the CoMo/SBA-15 catalyst may contribute to the enhancement of lignin degradation and the further deoxygenation of carbonyl (C]O) compounds by decarboxylation reactions, leading to the formation of monomers with a lower O content. The bands observed at 1602 and 1512 cm −1 can be attributed to the stretching vibration of C-H bonds in the aromatic skeleton, indicating the presence of aromatic compounds. These bands were found to increase in the catalytic bio-oils, which is consistent with the 13 C NMR results shown in Fig. 8b, where the Mo/SBA-15 and CoMo/SBA-15 catalyzed bio-oils showed high-intensity peaks of aromatic compounds (regions: d C = 100-140 ppm). In Fig. 7, the bands observed at 1270, 1150 and 1118 cm −1 were due to the stretching vibration of C-O bonds attached to the guaiacyl ring. Meanwhile the peak appeared at 1035 cm −1 is related to the C-O vibration in ether, acid or ester groups. As the non-catalytic bio-oil contained a higher amount of acid and ester compounds, the intensity of the peak at 1035 cm −1 was slightly higher compared to that of the bio-oils obtained with Mo/SBA-15 or CoMo/SBA-15 catalyst (Fig. 7b). In addition, the intensities of the peaks at around 860-741 cm −1 (assigned to the para-substituted aromatic ring) were found to increase in the CoMo/SBA-15 catalyzed bio-oil indicating that CoMo/SBA-15 produced a higher amount of para-substituted aromatic monomers in the bio-oil. These results are consistent with the GC-MS result presented in Fig. 6.
3.5.3 2D HSQC NMR analysis of Kra lignin and light-oil. The 2D-HSQC NMR method plays a vital role in elucidating lignin structures and characterizing lignin-derived bio-oil as it provides valuable insights into the structural characteristics of substructures and aromatic units in lignin and bio-oil. To investigate the structural changes in lignin before and aer catalytic reaction, 2D HQSC NMR was conducted. Fig. 9 shows the 2D HSQC NMR spectra of raw KL and the bio-oil obtained at 290°C for 4 h over CoMo/SBA-15 catalyst. The spectra are divided into three regions: the aliphatic hydrocarbon region (d C / d H 0-50/0.5-3.5 ppm), the oxygenated side-chain region (d C /d H 50-90/3.0-5.0 ppm) and the aromatic region (d C /d H 100-135/ 5.8-7.8 ppm). Peak assignments are provided in Table S4 † and Fig. S8-S10 of the ESI, † according to previous studies. 49,50 In the aliphatic hydrocarbon region (d C /d H 0-50/0.5-5.5 ppm), characteristic cross-peaks corresponding to methyl (-CH 3 ), methylene (-CH 2 -), and methine (-CH-) groups were found to be appeared to some extend in Kra lignin (Fig. 9a). However, noticeable differences were observed when comparing these cross-peaks with those of catalytic bio-oil. Most of the cross-peaks observed in the aliphatic region of the catalytic biooil were new ( Fig. 9(a 1 )), which can be attributed to the formation of phenolic-like ring structures with ortho-, meta/paramethyl, ethyl and propyl substituents. This nding is consistent with the GC-MS results of the bio-oil obtained with the CoMo/ SBA-15 catalyst (Fig. 6), which indicates a high content of alkyl-substituted aromatic monomers.
In  (Fig. 9b). The main inter-unit linkages in lignin are illustrated in Fig. 10. Aer catalytic depolymerization with CoMo/SBA-15 ( Fig. 9(b 1 )), the signals corresponding to A a , A b , B a, B b, B g , C b , and C g linkages in lignin were no longer observed in the bio-oil. This result suggests the near complete cleavage of C-O-C and C-C bonds in KL aer depolymerization with the CoMo/SBA-15 catalyst. Moreover, the signal of A g was still present in the spectra of the catalytic biooil, which can be attributed to the production of benzylic alcohol containing an OH group at the g-position. On the other hand, the signal of methoxy groups (-OMe) (d C /d H = 55.83/3.75 ppm) was found to be increased in the catalytic bio-oil. This result corresponds well to the lignin monomer products of GC-MS results (Fig. 6), in which high amount of methoxylated phenols were identied.
In the aromatic region (d C /d H 100-135/5.8-7.8 ppm) of the HSQC spectrum of KL (Fig. 9c) 4 . A minor C 2,6 -H 2,6 correlation from H units was detected, indicated by a signal at d C /d H 129.13/7.14 ppm. However, no signal for S unit was observed in the HSQC spectrum of lignin, suggesting that the KL used belonged to GH-type lignin. Upon the treatment of KL with a 10 wt% loading of CoMo/SBA-15 at 290°C for 4 h in ethanol, a brown oily lignin product was obtained, exhibiting an increase in the intensity of G-type units (Fig. 9c 1 ). These results are consistent with the FT-IR results (Fig. 7a), where the intensity of the G ring band in KL exhibited a lower peak intensity compared to that of the catalytic bio-oils.
3.5.4 Elemental analysis of heavy-oils. Elemental analysis was conducted on raw KL and the bio-oils obtained with or without catalysts and the results are summarized in Table S5. † It can be seen that KL has higher oxygen (O), nitrogen (N), and sulfur (S) contents than KL-derived bio-oils. For example, the O, N and S contents of KL were 30.09%, 0.65% and 1.48%, respectively. Aer KL depolymerization, these values decreased to 26.23%, 0.39% and 1.05% in the heavy-oil fraction of non-catalytic bio-oil, resulting in a relatively high HHV value of 28.44 MJ kg −1 (  (Table S5, † entry 5). The light-oil fraction showed a higher C (71.23%) and H (8.31%) content, and lower O (20.16%) and S (0.07%) contents, resulting in a higher HHV value of 32.60 MJ kg −1 compared to that of the heavy-oil fraction (29.75 MJ kg −1 ). These results reveal that CoMo/SBA-15 is an excellent catalyst for hydrogenation, hydrodesulfurization and deoxygenation during the lignin depolymerization in supercritical ethanol.
The O/C and H/C ratios of KL and the heavy-oil fraction of bio-oils are shown in Fig. 11A, while the S/C and H/C ratios are shown in Fig. 11B. It can be seen that the O/C and H/C ratios of KL were 0.488 and 0.099, respectively. Aer the depolymerization of KL at 290°C for 4 h (control experiment), the O/C ratio decreased to 0.401, and the H/C ratio increased to 0.105. Notably, in the presence of the CoMo/SBA-15 catalyst, the O/C ratio decreased and the H/C ratio increased signicantly. This nding indicates that the bimetallic CoMo/SBA-15 catalyst could enhance the hydrogenation of unsaturated bonds in lignin and lignin-derived depolymerized products, leading to an increased H/C ratio in the heavy-oil fraction. Furthermore, the O/C and H/C ratios of light-oil fraction obtained with the CoMo/ SBA-15 catalyst were also measured. It is noteworthy that the CoMo/SBA-15 catalyst yielded a light-oil with a higher H/C ratio and a lower O/C ratio, as well as a higher HHV value. On the other hand, the S/C and H/C ratios of KL were 0.024 and 0.099, respectively (Fig. 11B). However, the S/C ratio was found to be decreased in the heavy-oil and the light-oil fractions of bio-oil to 0.12 and 0.001, respectively, in the presence of CoMo/SBA-15 catalyst. The above ndings suggest that the CoMo/SBA-15 catalyst promotes the hydrogenolysis of lignin into various monomeric products and the further hydrogenation and deoxygenation of those products, resulting in the formation of saturated phenolic monomers, facilitated by the highly dispersed metal sites and acid active sites of the CoMo/SBA-15 catalyst.

Reaction route and mechanism
According to the XPS results, the Co existed in the form of (+3 and +2) oxidation states while Mo existed as +6 and +5 oxidation states (Fig. 3). The variable electronic valences of Co and Mobased catalysts make them promising catalyst in various heterogeneous catalytic reactions. In this study, to gain a better insight into the interaction between KL and bifunctional CoMo/ SBA-15 catalyst, the possible catalytic reaction mechanism is proposed in Scheme 1. Under supercritical condition, lignin was rstly decomposed into different reaction intermediates or macromolecular fragments by the solvolysis (e.g. ethanolysis). Subsequently, the macromolecular fragments may undergo various reactions such as hydrogenolysis, dehydration and decarboxylation, leading to the production of different derivatives of phenolic compounds. In the presence of catalyst (i.e. Scheme 1 Possible reaction routes during lignin depolymerization over the CoMo/SBA-15 catalyst. CoMo/SBA-15), the alcoholic solvent expected to produce active H* and ethoxy species (Scheme 1a). At the same time, the ethoxy group may activate saturated Mo species, resulting in active Mo sites. On the other hand, the dissociated active H species adsorbed on the adjacent terminal oxygen of MoOx species to form hydroxyl groups. Thereaer, an intramolecular proton transfer reaction may undergo to generate H 2 O molecule. Finally, the generated H 2 O species removed from the Mo surface to form active Mo sites with oxygen vacancy (i.e. unsaturated coordination). The active Mo species with oxygen vacancy then attacked the C-O bonds of macromolecular fragments (Scheme 1b) to generate intermediates products and further the intermediate products attacked by the many dissociative hydrogen species and transformed into phenolic derivatives through unsaturated side-chain hydrogenation and deoxygenation/dehydration reactions. The formation of unsaturated coordination in Mo species was also conrmed in Guo et al. 51 work, who investigated catalytic depolymerization of KL and hydrodeoxygenation of anisole (lignin-derived model compound) over MoOx/ZIF-8@ZIF-67 catalyst. They reported the formation of unsaturated coordination site (oxygen vacancy) in Mo catalyst and further the adsorption of C-O bonds of KLderived intermediates or anisole onto the active Mo species. The overall results reveal that under supercritical ethanol, the main mechanisms for the formation of different aromatic phenolic monomers over CoMo/SBA-15 catalyst were solvolysis/ hydrogenolysis, side-chain hydrogenation and deoxygenation (through dehydration) reactions.

Conclusion
In this contribution, we prepared monometallic Mo/SBA-15, Co/SBA-15 and bimetallic CoMo/SBA-15 catalysts via the wet impregnation method for the depolymerization of KL in supercritical ethanol. Our results showed that the CoMo/SBA-15 catalyst outperformed the Mo/SBA-15 and Co/SBA-15 catalysts in terms of depolymerization efficiency and the selective production of phenolic monomers from lignin. The presence of metal active sites in the CoMo catalyst and acid active sites in the SBA-15 support synergistically contribute to the cleavage of C-O-C and C-C bonds in lignin, resulting in a maximum phenolic monomer yield of 27.04 wt% at 290°C and 4 h. We found that KL can be effectively depolymerized in supercritical ethanol with the CoMo/SBA-15 catalyst via hydrogenolysis, hydrogenation and subsequent deoxygenation reactions. GC-MS and FT-IR analyses of bio-oil showed that the absence of a catalyst led to the formation of carbonyl compounds such as vanillin, ethyl vanillate and homovanillic acid in ethanol. In contrast, more alkylated phenols were produced in ethanol over CoMo/SBA-15 catalyst. The major phenolic monomers identied in the catalytic bio-oil were 4methylguaiacol, 4-ethylguaiacol and 4-propyl guaiacol. Furthermore, elemental analysis conrmed a reduction in the O content of the catalytic bio-oil, indicating the excellent hydrogenation and reductive catalytic activity of the CoMo/ SBA-15 catalyst in ethanol.

Conflicts of interest
There are no conicts to declare.